lEbe  'ClnfveteftB  of  Cbtcago 

FOUNDED  BY  JOHN  D.  ROCKEFELLER 


A NEW  METHOD  OF  DETERMINING  THE 
VAPOUR-DENSITY  OF  METALLIC  VAPOURS 


A DISSERTATION 

SUBMITTED  TO  THE  FACULTIES  OF  THE  GRADUATE  SCHOOLS  OF  ARTS, 
LITERATURE,  AND  SCIENCE,  IN  CANDIDACY  FOR  THE 
DEGREE  OF  DOCTOR  OF  PHILOSOPHY 

(department  of  physics) 


BY 

FRANK  B.  JEWETT 


CHICAGO 

1902 


i 

i 


#1 


From  the  Philosophical  Magazine /or  November  1902, 


A new  Method  of  determining  the  Vapour - Vensity  of  Metallic 
Vapour Sy  and  an  Experimental  Application  to  the  Gases  of 
Sodium  and  Mercury » By  FrakjS:  B«  Jewett. 

IN  all  investigations  on  the  composition  and  distribution  of 
light  in  the  speetral  lines  it  is  of  prime  importance  that 
the  lines  themselves  be  as  narrow  and  sharply  defined  as 
possible  ; this  is  especially  true  in  those  cases  where  the 
analysis  is  carried  on  by  means  of  interference  phenomena, 
for  here  the  difference  of  path  over  which  interference  takes 
place  decreases  as  the  width  of  the  line  increases. 

There  are  in  general  two  causes  which  may  affect  the 
breadth  of  the  lines  : (a)  motion  of  the  light-producing 

molecules  in  the  line  of  sight,  and  [h)  change  in  the  period 
of  the  souree  caused  by  frequent  collisions  of  the  molecules  *. 
To  these  might  be  added  a third  cause,  suggested  by  Lommel  f, 
in  which  an  inhomogeneity  is  produced  in  the  source  by 
forced  changes  in  the  period  of  ionic  vibration,  thus  putting 
an  upper  limit  on  the  power  to  produce  interference-fringes  ; 
this  latter  supposition  is,  however,  yet  to  be  verified,  and  Irom 
the  present  experimental  data  it  seems  probable  that  any 

^ Michelson,  Phil.  Mag.  (5)  xxxiv*  p.  293. 

t Lommel,  Wied.  Ann,  iii.  p.  251 ; Driide,  Lehrbuch  der  Optik,  p.  498. 


p 11152 


547  Mr.  F.  B.  Jewett  on  a Neiv  Method  of 

effect  due  to  it  must  be  almost  if  not  wholly  negligible  in 
comparison  with  that  of  (a)  and  (?>).  This  being  the  case, 
the  determination  of  the  relative  importance  of  the  two 
factors,  pressure  and  temperature,  is  the  question  which  at 
once  presents  itself  for  solution.  In  his  very  exhaustive 
article  on  ‘‘The  Application  of  Interference  Methods  to 
Spectroscopic  Measurements^^  Professor  Michelson  has  taken 
up  this  problem  in  considerable  detail.  He  finds  that  in  the 
cases  where  the  density  of  the  vapour  is  very  low  the  effect 
of  changing  pressure  on  the  width  of  the  spectral  lines  is 
almost  wholly  negligible  ; for  hydrogen  this  is  true  even  in 
the  case  where  the  pressure  is  as  high  as  2 or  3 mm. ; in 
fact,  when  the  relation  between  the  breadth  of  the  lines  and 
1/P  (pressure)  is  plotted  the  influence  of  P is  seen  to  become 
vanishingly  small  at  about  5 mm.  In  summing  up,  Professor 
Michelson  states  as  follows  : — “ It  thus  appears  that  in  the 
case  of  hydrogen — and  probably  in  all  other  cases — the  width 
of  the  spectral  line  diminishes  toward  a limit  as  the  pressure 
diminishes,  which  limit  depends  upon  the  substance  and  its 
temperature  ; and  that  the  excess  of  width  over  this  limit  is 
simply  proportional  to  the  pressure.” 

As  mercury  and  sodium  are  both  readily  usable  in  vacuum- 
tubes,  the  foregoing  facts  would  suggest  them  at  once  as  the 
ideal  substances  for  an  experiment  on  the  effect  of  pressure 
and  temperature  on  the  broadening  of  the  spectral  lines.  As 
a preliminary  to  such  an  experiment  a knowledge  of  the 
densities  of  the  saturated  vapours  at  various  temperatures  is 
of  coin-se  necessary.  In  addition  to  making  a determination 
of  the  densities  for  such  a purpose  as  the  above,  there  is  still 
another  and  even  more  urgent  reason,  viz.,  the  evident 
dependence  of  the  change  in  the  lines  in  the  Zeeman  effect, 
and  also  in  some  cases  the  reversal  of  the  same,  upon  the 
density  of  the  light-producing  vapour.  It  was  particularly 
with  a view  to  the  solution  of  this  latter  problem  that  the 
following  experiment  was  proposed  and  undertaken. 

Apparatus. — The  method  employed  was  one  suggested  by 
Prof.  Michelson,  in  which  the  amount  of  vapour  filling  a 
known  volume  is  determined  by  finding  the  amount  of  con- 
densed metal  in  the  observing  flask  when  the  latter  is  cooled 
off.  The  apparatus  consisted  of  three  essential  parts — the 
heating-batb,  the  gas-bulb,  and  the  thermometer.  The  bath 
finally  found  most  satisfactory  is  shown  in  section  in  fig.  1 
(p.  548) ; a and  b are  two  sheet-iron  boxes  lined  inside  and 
out  with  heavy  sheet-asbestos,  and  having  a 3-inch  air-space 
between  them  ; the  inner  box  [a)  is  about  14  inches  on  a side  ; 
around  the  inside  and  on  the  bottom  of  a are  a number  of  iron 


determining  the  Vapoi(,r~Density  of  Metallic  Vapours*  548 

resistance-coils  carried  on  an  asbestos-covered  iron  frame, 
and  ending  in  two  heavy  terminal  wires  cc,  which  pass  out 


Fig.  1. 


a metal  frame  e^  and  constant  circulation  is  maintained  by 
an  electrically-driven  fan  f ; g represents  the  stem  of  a 
platinum  thermometer.  With  this  arrangement,  and  with 
suitable  regulation  for  the  current,  the  temperature  may 
easily  be  kept  constant  at  any  desired  point  to  within  one  or 
two  degrees*. 

The  gas-bulb  or  reservoir  (fig.  2)  was  of  hard  glass  of 
known  cubical  content,  and  had  a capacity  of  about  2000  c.c. ; 
proceeding  from  the  bulb  were  two  tubes,  one  a heavy 
capillary  and  the  other  with  an  internal  diameter  of  about 
1 cm. 

* In  one  instance  where  the  bath  was  used  for  calibrating  a Beckmann 
thermometer  the  temperature  was  held  constant  to  l for  fifteen 
minutes. 


549 


Mr.  F.  B.  Jewett  a New  Method  of 


The  thermometer  was  of  the  Callendar  platinum-resistance 
type  with  auxiliary  compensating-leads  and  direct  reading 


Fig.  2. 


Wheatstone  bridge,  and  was  capable  of  reading  to  0^*005  ; 
this  particular  instrument  was  one  of  those  calibrated  at  ihe 
Kew  Observatory. 

In  order  to  make  an  observation  a tube  (a,  fig.  2)  was 
sealed  to  the  larger  tube  (/>),  and  the  capillary-tube  (c)  drawn 
down  at  [d)  ; this  being  done,  and  both  tubes  and  bulb 
thoroughly  dried,  a small  piece  (0*5-0‘7  gm.)  of  C.P.  metallic 
sodium  was  introduced  into  (a),  and  the  latter  quickly  sealed 
off  at  {e)^  as  shown  ; (c)  was  now  connected  to  a Geissler- 
pump,  and  the  air  drawn  out  to  a residual  pressure  of  0*1- 
0*2  mm.,  after  which  the  bulb  was  filled  with  some  inert  gas 
(H  or  N)  and  again  pumped  out  and  the  capillary-tube 
sealed  off  at  [d) . The  bulb  thus  prepared  was  now  introduced 
into  the  bath  and  the  temperature  raised  to  any  required 
point  ; the  apparatus  was  kept  at  the  desired  temperature  for 
fifteen  or  twenty  minutes,  thermometer-readings  being  taken 
every  two  minutes ; the  cooling  had  to  be  done  very  slowly,  as 
the  capsule  containing  the  molten  sodium  was  very  liable  to 
crack,  and  the  inrushing  air  carried  the  metal  into  the  bulb. 
Upon  removing  the  latter  from  the  bath,  the  whole  inner 
surface  showed  a bright  metallic  coating  of  condensed  sodium 
vapour,  varying  in  thickness  with  the  temperature  to  which 
the  bulb  had  been  subjected.  To  determine  the  amount  of 


* E.  H.  Griffiths,  ‘ Nature,’  Nov.  14,  1895. 


determining  the  Vapour-Density  of  Metallic  Vapours,  550 

soJium  in  this  coating,  and  consequently  the  amount  of 
saturated  vapour  that  had  filled  the  bulb,  the  tube  {h)  was 
cracked  off*  at  some  point  (m),  thus  getting  rid  of  the  metal 
remaining  in  {a) ; the  bulb  was  then  thoroughly  washed  out 
with  hot  water  until  the  washings  failed  to  show  an  alkaline 
reaction  with  phenolphthaline,  and  the  amount  of  Na  present 
as  NaOH  in  the  washings  determined  by  differential  titration 
with  standardized  N/IO  • NaOH  and  N/10  * H2SO4  solutions  ; 
this  amount,  together  with  the  corrected  volume  of  the  bulb, 
furnished  the  requisite  data  for  finding  the  vapour-density. 
As  this  process  had  to  be  repeated  for  every  determination, 
the  making  of  a large  number  of  observations  was  an  ex- 
ceedingly tedious  matter. 

The  above-described  method  was  the  one  finally  chosen  for 
sodium  ; a number  of  methods  depending  upon  the  gravi- 
metric determination  of  the  amount  of  metal  volatilized, 
while  giving  good  results  for  those  metals  which  do  not 
oxidize  easily  at  low  temperatures,  e.  g.  Cd,  proved  absolutely 
useless  in  the  case  of  sodium  on  account  of  the  rapid  oxidiza- 
tion of  the  latter  when  in  contact  with  the  air. 

A difficulty  which  it  was  at  first  feared  might  render  the 
determination  impossible  at  the  higher  temperatures,  viz.,  the 
action  of  sodium  on  glass,  was  not  encountered  except  when 
the  residual  atmosphere  contained  0 or  water-vapour,  the 
solvent  action  being  apparently  exhibited  only  for  the  oxide 
or  hydroxide  ; nside  Irom  this  fact  the  results  obtained  in 
the  presence  of  air  were  so  extremely  erratic  that  all  the 
final  determinations  were  made  either  in  the  presence  of 
oxygen  or  nitrogen.  (The  majority  of  the  tests  were  made 
with  hydrogen,  and  as  they  gave  consistent  results  the 
accuracy  of  the  process  was  not  questioned  at  the  time, 
especially  as  the  hydride,  Na4H2,  was  not  supposed  to  form 
at  pressures  so  reduced  as  those  employed  Owing,  how- 
ever, to  a peculiar  brown  metallic  appearance  of  the  deposit 
in  some  instances,  doubt  was  cast  on  the  validity  of  this 
assumption,  so  that  while  the  great  mass  of  chemical  data 
seems  to  weigh  against  the  formation  of  the  hydride,  there 
still  remains  the  possibility  that  the  density  of  the  vapour, 
calculated  on  the  assumption  that  it  consisted  of  free  Na, 
gave  too  low  a result.  This  question  can  be  easily  settled, 
however,  by  the  employment  of  N,  since  the  nitride,  NaNs, 
is  not  formed  by  the  direct  combination  of  Na  and  N f*) 

Some  of  the  results  are  given  in  Table  I.,  and  the  curve, 

Eoscoe  & Scliorlemnier,  ^ Treatise  on  Chemistry/  vol.  ii.  pt.  i.  p.  107. 

t Berichte,  xxv.  p.  2084  (1892) ; Zeit.f.  anovg.  Ch,  vi.  p.  38  (1894). 


551 


Mr.  F.  B.  Jewett  on  a New  Method  of 

witli  temperatures  as  ordinates  and  densities  as  abscissse,  is 
shown  in  fig.  3 (p.  552) ; the  dotted  curve  shown  is  that  for  Hg 
at  temperatures  where  the  density  of  the  vapour  corresponds 
to  that  of  Na  ; the  temperatures  for  this  latter  curve  are 
indicated  on  the  curve. 


Table  I. 


Temp, 

Density. 

368 

000000009 

373 

00000002 

376 

000000035 

380 

0-00000043 

385 

0-00000103 

387 

0-00000135 

300 

0-00000160 

395 

0-00000270 

400 

0 00000350 

406 

0 00000480 

408 

0-00000543 

412 

0 00000590 

418 

0 00000714 

420 

0-00000750 

While  the  densities  were  not  obtained  much  below  365^, 
it  will  be  noticed  that  at  this  temperature — which  cannot  be 
far  from  that  commonly  employed  in  vacuum-tube  work  * — 
the  density  of  the  Na  vapour  increases  at  about  the  same  rate 
as  that  of  Hg  at  85*°,  while  at  points  slightly  above  this  the 
Na  curve  increases  much  the  more  rapidly.  This  fact,  taken 
together  with  its  low  atomic  weight,  might  well  account  for 
the  peculiarities  observed  by  Professor  Michelson. 

As  was  stated  above,  the  experiment  was  undertaken  solely 
to  determine  the  densities  within  the  range  betw^een  350^  and 
450°,  and  indeed  the  use  of  a glass  bulb  precludes  the 
possibility  of  anything  being  done  above  500^ ; with  a 
porcelain  bulb  it  would  be  comparatively  easy  to  attain  any 
desired  temperature  below  1700°. 

The  ease  with  which  the  temperature  of  the  electric  bath 
could  be  regulated  at  once  suggested  the  desirability  of 
making  a series  of  determinations  on  the  vapour-density  of 
Hg,  and  with  the  slight  alteration  in  the  form  of  the  bulb 

* Professor  Michelson  assumes  that  the  temperature  of  the  heating- 
box,  350°,  is  that  of  the  vapour  also.  There  appears  to  be  some  doubt  as 
to  the  legitimacy  of  this  assumption,  owing  to  the  very  considerable 
heating  produced  by  the  discharge  itself. 


determining  the  Vapour-Density  of  Metallic  Vapours.  552 


3. 


553 


Mr.  F.  B.  Jewett  on  a New  Method  of 

shown  in  fig.  4,  a set  of  continuous  readings  5°  apart  was 
obtained.  The  mercury-reservoir  (a,  fig.  4)  is  a long  narrow 

Fig.  4. 


tube  of  hard  glass  of  known  coefficient  of  expansion,  and 
having  a carefully  calibrated  bore.  In  making  a deter- 
mination the  reservoir  is  charged  with  a known  weight  of 
mercury,  and  the  bulb  exhausted  and  sealed  off  as  in  the  case 
of  sodium  ; it  is  then  introduced  into  the  bath  and  supported 
in  such  a position  that  (a)  is  vertical  ; the  height  of  the 
mercury  column  is  observed  through  glass  windows  in  the 

Table  II. 


Temp. 

Density. 

Regnault  & Hertz. 

Ramsay  & Young. 

o 

40 

0-00000007 

0-00000007 

0 00000009 

60 

00000003 

0-0000003 

0-0000003 

70 

000000045 

0-0000005 

0 0000005 

80 

0-0000007 

0-0000008 

0 0000008 

90 

0-0000012 

0-0000014 

0 0000014 

100 

0-0000021 

0-0000024 

0-0000023 

110 

0 0000040 

0-0000039 

120 

00000060 

0-00r;0064 

0-0000059 

140 

0-0000138 

0-0000147 

0 0000137 

160 

0-0000302 

0-0000323 

0-0000297 

180 

0-0000624 

0-0000649 

0 0000603 

200 

00001580 

0-0001236 

0-0001152 

220 

0-0002020 

0-0002271 

0-0002077 

240 

0-0003754 

0-0003673 

...  . . 

260 

0-0005830 

00005817 

270 

0-0006528 

0-0007257 

0-0007310 

280 

0-0008645 

0-0008994 

0-0009113 

300 

00013466 

0-0013547 

0-0013796 

305 

0-0013882 

310 

0-0016447 

0-0016472 

0-0016734 

320 

0-0019879 

0-0019921 

0-0020180 

325 

0-0019960 

determining  the  Vapour- Density  of  Metallic  Vapours,  554 

sides  of  the  bath  by  means  of  a cathetometer.  The  readings 
thus  obtained,  together  with  the  known  coefficients  of  ex- 
pansion of  glass  and  mercury,  furnish  the  requisite  data  for 
determining  the  amount  of  metal  volatilized.  A partial 
series  of  the  results  obtained  is  given  in  Table  II.,  together 
with  the  results  calculated  from  the  observed  tensions  of 
mercury-vapour  as  given  by  Eegnault  and  Hertz ; a more 
complete  set  of  readings  will  be  published  later. 

In  conclusion  I desire  to  express  my  thanks  to  Professor 
Michelson  for  the  encouragement  and  helpful  criticism  given 
throughout  the  work,  and  also  to  Dr.  Gale  for  the  assistance 
so  kindly  rendered  in  the  work  on  the  density  of  mercury- 
vapour. 

Kyerson  Physical  Laboratory, 

March  25, 1902. 


I 


U 


